1. Field of the Invention
The present invention relates to optical communication equipment and, more specifically but not exclusively, to signal processing that can be implemented at a coherent optical receiver of multicarrier offset-quadrature-amplitude-modulation (MC-OQAM) signals.
2. Description of the Related Art
This section introduces aspects that may help facilitate a better understanding of the invention(s). Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
Delivery of multimedia services (e.g., telephony, digital video, and data) that is implemented using optical phase-shift keying (PSK) or quadrature-amplitude modulation (QAM) signals has certain advantages, e.g., over that implemented using conventional electrical analog or digital signals. As a result, cable and telephone companies and Internet service providers are upgrading their networks to incorporate optical links that can carry these types of optical signals. For example, it is projected that, in the near future, high-definition television signals are likely to be delivered predominantly over optical communication channels.
A typical coherent optical receiver detects a received PSK or QAM signal by mixing it with a local-oscillator (LO) signal and then processing the resulting mixed signals to determine the phase and amplitude of the received signal in each time slot (symbol period), thereby recovering the encoded data. To enable the phase and amplitude determination, the LO signal may be phase-locked to a carrier frequency (wavelength) of the received optical signal using an optical phase-lock loop (PLL). More specifically, a PLL may be configured to track a carrier frequency of the received optical signal and provide a feedback signal to the LO source, based on which the LO source can achieve and maintain the phase lock.
Unfortunately, suitable coherent optical receivers are typically relatively difficult to design and/or relatively expensive to build. For example, a conventional, relatively inexpensive laser source might produce an optical signal that has a relatively large linewidth. If that laser source is used in a coherent optical receiver as a local oscillator, then its relatively large linewidth might produce a phase uncertainty and/or phase noise that can make an optical phase-lock between the LO and communication signals difficult to achieve and/or maintain. As another example, even a laser source having a relatively narrow linewidth may require a fairly complex (and therefore relatively expensive) PLL circuit to function as a local oscillator in a coherent optical receiver.